@NJG It takes a lot of time to code interactive fiction. As great as it would be to be able to add a zillion fun little extra options to the game, I’d rather try to keep things plot-relevant and actually finish the game someday (it’s taking long enough as it is). 
As far as arresting Arthur for infidelity goes (also @OtherGrimm and @FoxalypticWorld and @Fallaner)…
One way in which the Guenverse is different from “real” history is that any member of the royalty having hetero extramarital sex is considered treason against the state, because a child could be produced and the royal bloodlines thereby thrown into confusion – royal blood carries magical abilities, which will manifest for Arthur in part 4, and you don’t risk just anyone being born with that power. (That said, it’s an arbitrary cultural taboo as much as a practical one.) So Arthur having sex outside of marriage with any woman (or theoretically anyone who is perceived to have a uterus) does count as treason, as would Guen having sex with any man. (Same-sex affairs don’t fall under the same taboo.)
So, in theory, if Arthur were accused of extramarital hetero sex, and the accuser made a convincing case, the other nobles would have a reason to depose him. It would not be as simple as Guen calling the guards, having him arrested, and taking over. Guen is queen by marriage and not by blood; if Arthur were deposed, she’d lose her own status and power. The most likely scenario is that some opportunistic nobles would rally opposition against Arthur and raise up a king or queen from one of the other royal bloodlines (the most likely candidate being either Gawain, since he comes from the Lothian royal line, or one of the children of King Pellinore, who will appear in part 4). Assuming Guen survived the political upheaval, she’d end up back in Cornwall in disgrace.
Unless, of course, many years in the future, Guen has built up such a huge and loyal following that the vast majority of Britons support her as queen even without Arthur… or, perhaps, if she had already produced a legitimate heir so that the succession was not in doubt; or, just maaaaaybe, if an illegitimate heir to the Pendragon blood were to rally support, get rid of Arthur, and marry Guen. But in the nearer future, a threat to Arthur’s reign is a threat to Guen’s.